Hydraulically fracturing of subterranean formations to increase oil and gas production has become a routine operation in petroleum industry. In hydraulic fracturing, a fracturing fluid is injected through a wellbore into the formation at a pressure and flow rate sufficient to overcome the overburden stress and to initiate a fracture in the formation. The fracturing fluid may be a water-based liquid, an oil-based liquid, liquefied gas such as carbon dioxide, dry gases such as nitrogen, or combinations of liquefied and dry gases. It is most common to introduce a proppant into the fracturing fluid, whose function is to prevent the created fractures from closing back down upon themselves when the fracturing pressure is released. The proppant is suspended in the fracturing fluid and transported into a fracture. Proppants in conventional use include 20-40 mesh size sand, ceramics, and other materials that provide a high-permeability channel within the fracture to allow for greater flow of oil or gas from the formation to the wellbore. Production of petroleum can be enhanced significantly by the use of these techniques.
Since a primary function of a fracturing fluid is to act as a carrier for the introduced proppant, the fluids are commonly gelled to increase the viscosity of the fluid and its proppant carrying capacity, as well as to minimize leakoff to the formation, all of which assist in opening and propagating fractures. To allow for the formation to flow freely after the addition of the viscous fracturing fluid, chemicals known as breakers are added to the fracturing fluids to reduce the viscosity of the fluid after placement, and allow the fracturing fluid to be flowed back and out of the formation and the well.
The breaking of the fracturing fluid involves a complicated chemical reaction that may or may not be complete. The reaction itself may leave a residue that can plug the formation pore throats, or at very least reduce the effectiveness of the fracturing treatment. Many subterranean formations are susceptible to damage from the liquid or carrier phase itself, necessitating careful matching of fracturing fluids to the formation being fractured. Certain sandstones, for instance, may contain clays that will swell upon contact with water or other water-based fracturing fluids. This swelling decreases the ability of the formation fluids to flow to the wellbore through the induced fracture and therefore, inhibits or at very least reduces, the effectiveness of the fracturing treatment.
With specific reference to coalbeds, underground coal seams often contain a large volume of nature gas, and fracturing coal seams to enhance the gas production has become a popular and near-standard procedure in coalbed methane (CBM) production. Coal seams are very different from conventional underground formations such as sandstones or carbonates. Coal can be regarded as an organic rock containing a network of micro-fissures called cleats. The cleats provide the major pass ways for gas and water to flow to the wellbore. The cleats in coal, however, are very susceptible to damage caused by foreign fluids and particulates. Therefore, it is very important to use clean fluids in fracturing coal seams. High pressured nitrogen has been used in fracturing coal seams. Since it is gas and can be easily released from coal seams after the fracturing treatments, it causes very little damage to the formation.